cmd/compile: make sure build works when intrinsics are disabled

Some runtime functions, like getcallerpc/sp, don't have Go or
assembly implementations and have to be intrinsified. Make sure
they are, even if intrinsics are disabled.

This makes "go build -gcflags=all=-d=ssa/intrinsics/off hello.go"
work.

Change-Id: I77caaed7715d3ca7ffef68a3cdc9357f095c6b9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/179897
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -3658,9 +3658,6 @@ func init() {
 // findIntrinsic returns a function which builds the SSA equivalent of the
 // function identified by the symbol sym.  If sym is not an intrinsic call, returns nil.
 func findIntrinsic(sym *types.Sym) intrinsicBuilder {
-	if ssa.IntrinsicsDisable {
-		return nil
-	}
 	if sym == nil || sym.Pkg == nil {
 		return nil
 	}
@@ -3680,6 +3677,13 @@ func findIntrinsic(sym *types.Sym) intrinsicBuilder {
 	}
 
 	fn := sym.Name
+	if ssa.IntrinsicsDisable {
+		if pkg == "runtime" && (fn == "getcallerpc" || fn == "getcallersp" || fn == "getclosureptr") {
+			// These runtime functions don't have definitions, must be intrinsics.
+		} else {
+			return nil
+		}
+	}
 	return intrinsics[intrinsicKey{thearch.LinkArch.Arch, pkg, fn}]
 }